A wide variety of telephone users, residential as well as commercial, require telephone installations in which calls can be received at or placed from a number of different telephones using any one of a limited number of telephone lines from a central office or from a private branch exchange (PBX). In the past, the requirements for installations of this kind have generally been met with key telephone systems. But conventional key telephone systems are not economically attractive when used with small installations involving only a few telephone lines (e.g., four or less) and a limited number of telephone stations (ten or less).
Thus, a conventional key telephone system requires a central control that must be connected to each individual telephone by a multi-conductor cable. The central control and system wiring for the key telephone system is quite complex and relatively costly, particularly in relation to any local system involving no more than four outside lines and no more than ten telephones. Further, each key system telephone and control circuit is usually constructed with provisions for key functions to accommodate the maximum number of telephone lines to which that telephone and control might be connected in a variety of different applications. That is, a conventional key telephone system is usually provided with at least a five line capacity and often more, a substantial excess for a residential or business application having only two or three telephone lines and only a few telephones.
One arrangement for a plural line telephone control system specifically adapted for use in small local systems is described in Rasmussen U.S. Pat. No. 4,132,860 and in Rasmussen et al U.S. Pat. No. 4,133,985. The control described in those two patents affords many of the operating features of a conventional key telephone system in an installation that provides for connection of a station control unit or converter between each telephone instrument and a limited number of telephone lines, usually no more than four lines. Each converter, like a conventional key telephone system instrument, provides a selector switch for each telephone line, together with a hold switch and hold circuits to permit a call to be placed in a hold condition. The system further includes a conference call arrangement that can be effected by simultaneous operation of two or more of the line selection switches.
The control apparatus described in the Rasmussen patents, however, requires special wiring between the converters and the telephone instruments; the bell or other audible signal device in each telephone must be disconnected from the normal actuation circuits in the telephone and re-connected to the control unit (converter) for that telephone. The control units, although equipped with indicators to show some conditions for the trunks and for the telephones, do not provide a full range of condition indications to enable a telephone user to determine all relevant operating conditions at any given time. The converters or control units are not readily adaptable to an increase in the number of telephone lines connected to the local system when such expansion is required. Moreover, the control units described in these patents require a substantial number of relays, at least one of which is continuously energized, and hence may have an undesirably high power consumption level.